The present invention relates to loudspeaker systems for high fidelity stereo sound reproduction.
It is a goal of high fidelity sound reproduction to faithfully reproduce and present to the listener the sounds of the original acoustic event with the proper phase relationships over the audible range.
Loudspeakers generally incorporate a vibrating diaphragm which is driven by an electronic signal to produce audible sound waves. This arrangement presents several problems. For high fidelity sound reproduction over the audible range a speaker diaphragm ideally moves in a piston-like motion where the entire surface vibrates in phase. To effectively radiate low frequencies a large speaker diaphragm excursion is generally required. This is usually achieved with a loudspeaker having a large diaphragm. Because of the large diaphragm such speakers are prone to speaker breakup at high frequency. Speaker breakup occurs when different portions of the diaphragm vibrate out of phase with the resulting sound also being out of phase.
Consequently many speaker systems employ a dual loudspeaker arrangement of large diaphragm "woofers" to transmit low frequency sound and small diaphragm "tweeters" to transmit high frequency sound. In some systems the transition range between low and high frequency sound is covered by additional mid range speakers. A switching network is required to prevent high frequency signals from reaching and breaking up the low frequency woofers.
Because of the poor response characteristics of each type of speaker over a portion of the audible range and the necessity of an electronic switching network to prevent speaker breakup, smooth frequency response over the full audible range is impaired.
The use of diaphragm loudspeakers creates sound radiation from both faces of the diaphragm. The sound radiation from the front of the diaphragm is 180.degree. out of phase from that of the rear of the diaphragm. Unless these radiations are isolated, destructive interference and a decrease in speaker efficiency will result.
The simplest way of achieving this isolation is to place the speaker in an infinite baffle, i.e. a sheet of acoustically non-conducting material of sufficiently large dimensions to prevent radiation on one side of the baffle from reaching the other side. While in theory this works well, the large dimensions involved make it impractical for most applications. Attempts to approximate the effect of an infinite baffle include placing a speaker in an enclosure which exposes only the front face of the speaker to the external environment. The radiation from the rear of the diaphragm is confined to the enclosure. Because the enclosure possesses its own acoustic resonance and the radiation from the rear of the diaphragm is coupled to the speaker through the enclosure, the isolation of the front and rear radiation is degraded. This problem can be minimized by lining the box with sound absorbing material to absorb the radiation from the rear of the diaphragm. This solution is undesirable because it results in greatly lowered acoustic efficiency as the radiation of all frequencies from the rear of the diaphragm is lost.
This loss can be minimized if the radiation from the back side of the diaphragm is directed to the external environment in such a manner so as to be in phase with the sound radiation from the front of the diaphragm. This may be achieved by incorporating a port in the speaker enclosure and choosing the dimensions of the enclosure such that the port radiates sound in phase with the front side radiation from the diaphragm.
The prior art in the stereo sound reproduction field has established that an in phase, uniform and smooth frequency response over the range of audible frequencies is desirable for good stereo image perception. The desirability of retaining such in phase, uniform and smooth frequency response over a wide dispersion angle for good stereo image perception is also well established.
Multiple speaker systems are commonly used in stereo sound reproduction systems to achieve stereo image perception. Here the goal is to produce stereo image perception while minimizing directional dependence which might result in confusing sound images. For this purpose, it is desirable to have the acoustic energy dispersed in a vertical cylindrical wavefront if the loudspeaker system is in the center of the audience, or in a more common case, as a vertical hemicylindrical wavefront if the loudspeaker is mounted against a wall. This point is discussed in U.S. Pat. No. 3,668,335. Here the loudspeaker gives the acoustic appearance of a narrow vertical slot in a wall, radiating sound over a wide angle in the horizontal plane with minimal angular dependence of the sound intensity while minimizing sound transmission to, or reflection from horizontal surfaces such as the ceiling and floor.
One approach to this goal is shown in U.S. Pat. No. 3,668,335 and U.S. Pat. No. 3,980,829 which show the use of an acoustic lens. The lens is a rather large and complex structure and the speaker system does not utilize the backside radiation of the sound transducer. Linear arrays of loudspeakers have been used in several systems. U.S. Pat. No. 3,299,206 shows a linear array incorporating three different types of loudspeakers together with sound absorbing material to moderate the directivity of the output. This arrangement also does not use the backside output of the speakers. U.S. Pat. No. 4,267,405 shows a vertical column of loudspeakers including high and low frequency speaker assemblies. This system includes a filter network to separate high and low frequency output and prevent speaker breakup. Again, the backside output of the speakers is not used.
In view of the above, it is evident that there exists a need for a loudspeaker system having an increased acoustic efficiency, which utilizes the frontside and backside output of the diaphragm and which provides good stereo image perception. It is an object of this invention to provide a loudspeaker system with an increased acoustic efficiency. It is a further object to provide for increased acoustic efficiency by utilizing a plurality of small diameter speaker units, all of which are in phase. It is a further object to provide a loudspeaker system which provides an intense sound radiation output at the listening level so as to provide increased stereo image perception while preventing sound radiation upwardly or downwardly. A further object of this invention is to provide these features in a structure having a size which makes it practical for in home use. It is a further object of this invention to provide these features in a structure which is easily disassembled into smaller units for ease of transportation or shipping. It is a further object of this invention to provide a loudspeaker system having these features in a simple structure which can be constructed economically from readily available components.